In recent years, there is a request for a fuel injection device an engine is required to suppress a total amount of particulate matters (PM) in an engine during a mode travel and the particulate number (PN) which is the number of particulate matters as exhaust regulations are reinforced, and can control a minute injection amount. As a method of suppressing the particulate matters, it is effective to split spraying in one combustion stroke into a plurality of times for injection (hereinafter, referred to as a split injection). With the split injection, adhesion of fuel to a piston and a cylinder wall surface can be suppressed. Therefore, the injected fuel can be easily vaporized, and the total amount of the particulate matters and the particulate number (the number of particulate matters) can be suppressed. The engine performing the split injection needs to split one fuel injection so far into a plurality of times for the injection. Therefore, in the fuel injection device, there is a need to control a fine injection amount compared to the related art. In addition, in a multi-stage injection, the particulate number can be easily suppressed by increasing the number of times of injection. Therefore, the fuel injection device is required to be improved in responsiveness and to reduce an interval of fuel injections.
In general, the injection amount of the fuel injection device is controlled by a pulse width of an injection pulse which is output by an engine control unit (ECU). In a normally-closed magnetic fuel injection device, there is a bias member which generates a force in a closing direction. A drive unit is configured by a coil, a fixed core, and a movable element. A magnetic attraction force is generated between the fixed core and the movable element by supplying a current to the coil. The movable element moves in the opening direction at the time when the magnetic attraction force exceeds the force in the closing direction. The valve is separated from a valve seat and starts to open at timing when the movable element comes into conflict with a valve. When the current supplying to the coil is stopped after the valve is opened, the attraction force generated between the fixed core and the movable element is lowered, and the valve starts to be closed at a time point when the force becomes smaller than a force in the closing direction.
In general, a drive circuit of the magnetic fuel injection device applies a high voltage from a high-voltage source to the coil at the beginning when the injection pulse is output in order to make the valve to move quickly from a closed state to an open state, and controls the current of the coil to steeply rise up. Thereafter, the movable element is separated from the valve seat, and moves in the direction of the fixed core. Then, the voltage is switched to a low voltage to control a switch to supply a constant current to the coil. In a case where the current supplying to the coil is stopped after the movable element comes into conflict with the core, the movable element is delayed in opening. Thus, a controllable injection amount is restricted. Therefore, the current supplying to the coil is stopped before the movable element comes into conflict with the fixed core. The valve is required to be controlled on a condition that the movable element and the valve move parabolically (that is, a half lift).
In addition, on the condition of the half lift, the displacement of the movable element is not restricted to the fixed core, and thus the operation becomes unstable. The operation is easily influenced by a change in environmental condition such as an applying voltage of the coil.
For example, PTL 1 discloses a method of controlling the change of the environmental condition. In PTL 1, a maximum current of a drive current flowing to a drive coil of the fuel injection device is changed according to a driving condition to secure the stability of the movable element and to reduce a deviation of the injection amount.